King Miro was once the sovereign of Eternia, and sworn enemy of a witch who called herself Enchantress. One day, he was captured by his nemesis—possibly in the Swamps of Enchantment, as that is where a merchant would one day discover a bracelet with his crest upon it—and imprisoned at the top of Lost Mountain, a peak atop a mysterious floating island.

With the king gone, his son—Randor—was anointed ruler of Eternia. For many years, Miro remained the prisoner of Enchantress, until King Randor’s Man-At-Arms discovered the former ruler’s bracelet at a bazaar. This fresh clue, after years of fruitless search, was just what King Randor needed. He set off at once with Man-At-Arms, followed shortly thereafter by He-Man at the behest of Queen Marlena, to find his Miro.

Eventually, the heroes found the old king, but Enchantress wasn’t about to let them flee with her prisoner so easily. She used her magic to enlarge her servant, Drude, and set him against them. Miro turned the tables on her though, by convincing Drude that he would never be anything more than her slave if he continued to bow to her wishes. Fortunately for Miro and his companions Drude agreed, and he turned on Enchantress.

Later, once everyone was safe, Randor offered to return the crown to his father, but Miro refused, feeling that he had lost too much time and desiring to reconnect with the people of Eternia.

Some time later, Miro returned to the Palace of Eternia, excited by the news that his granddaughter has been found on the planet Etheria. An eager Prince Adam escorted Miro to Castle Grayskull, where they used one of the Sorceress’ travel corridors.

Upon their arrival on Etheria, they thwarted an ambush by some Horde Troopers, and fled into the safety of Whispering Woods, where Miro was finally reunited with Princess Adora. The happiness of their reunion was short-lived, however, as Horde Inspector Darkney—with Hordak and his warriors at his side—erected a powerful crystal dome over the woods, trapping the Great Rebellion within. While Miro, Madame Razz, and Broom challenged Hordak, his grandchildren transformed into He-Man and She-Ra, and set about destroying the means of their imprisonment.

Later, with the Horde defeated once more, Miro and Prince Adam said their goodbyes, but not before vowing to return to Etheria for the final battle.

Bio #2 (MYP 200X):

Because the 2002 animated series presented Randor as a common man who is named king by the Council of Elders, Miro was never king in this version of continuity. Instead a character appears named Captain Miro, implied to be the same person.

Bio #3 (MOTUC Toyline):

The Masters of the Universe Classics toy line presents Miro as king and leader of the "House of Miro," with Keldor and Randor as his sons. Hence, Miro is also the father of Skeletor.